I gone in 2003 and 2004, and attended two parties each year. 2003 was much better in terms of characters, I thought, as far more villains were out! 2004 was OK, but I had much more fun meeting characters in 2003!
Anyways, in 2003 we met (I am just typing this off the top of my head!):
Snow White Witch, Pain, Panic, Scrooge, Big Bad Wolf, Fairy Godmother, Cinderella, Radcliffe, Sheriff of Nottingham, Jafar, Foulfellow, Stromboli, Frollo, Cruella, Maleficent, Goliath the Gargoyle, Zowie, Rolie Polie Olie, Doc, Sleepy, Grumpy, Dopey, Happy, Bashful, Sneezy (and autographs from all dwarves!), Snow White, Phoebus, Clopin, Esmeralda, Quasimodo, Aladdin, and the classics (Mickey, Minnie, Donald, Goofy, Pluto, Chip, Dale, Pooh, Eeyore and Tigger).
In 2004, we went to the last two Halloween parties in October. There we saw: Terk, Scrooge, Daisy, Darkwing Duck, Thumper, Sora (from Kingdom Hearts), Cinderella and her Prince, Aurora, Goliath the Gargoyle, Goliath the Lion, Jojo, Esmeralda, Quasimodo, Mulan, Mushu, Maleficent, Cruella, Jafar, Fairy Godmother, Suzy and Perla Mouse, Frollo, Big Bad Wolf, Walrus, Queen of Hearts, Alice, White Rabbit, Mad Hatter, Tweedle Dee and Dum, Stitch, Snow White Queen, Snow White, all seven dwarves (group pictures only not autographs), Aladdin and Jasmine, and the classics (Mickey, Minnie, Goofy, Pluto, Donald, Chip, Dale, Pooh, Eeyore, Tigger, and Piglet).
Whew! Looks like an impressive list! I am probably forgetting a character here and there, but you can get a rough idea of who's out and about!

October has become my favorite time of year to go to Disney, and the Halloween parties are the main reason why!
Happy character hunting!